Common examples of powder coatings include epoxy-series, acryl-series and polyester-series. Polyester-series powder coatings, particularly those which are hardened with isocyanate and mainly comprise polyester having a hydroxyl group at a terminal end, have been known to provide well-balanced coating films excellent in weatherability, corrosion resistance and mechanical strength.
Powder coatings have been widely used in various fields including appliance, automobile and construction materials. In some cases, a glossy coated surface (specular gloss at 60.degree.: approximately 90 to 100%) is required for aesthetic purposes, while a matte (specular gloss at 60.degree.: less than 30%) or semimatte (specular gloss at 60.degree.: 30 to 70%) may be required for other purposes.
Examples of conventional methods for preparing such a matte or semimatte powder coating include a method comprising adding a large amount of a pigment comprised of coarse particles, and another method comprising dry-blending a polyester powder coating with an acryl powder coating as disclosed in JP-B-61-19668 (the term " JP-B" as used herein means an "examined published Japanese patent application").
However, the addition of a large amount of a pigment of coarse particles causes a deterioration in smoothness or a decrease in mechanical strength of the coating film. On the other hand, when using the method of dry-blending the two powder coatings, a matte or semimatte powder coating cannot be obtained by a single step, which increases production cost. Furthermore, this method suffers from other problems including heterogeneity of the resulting blend and a change in the blending ratio during recovery and application.
In order to solve the above-mentioned problems, JP-A-64-1770 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a resin composition for a powder coating comprising a polyester having a hydroxyl value of at least 1,200 g eq/10.sup.6 g, another polyester having a hydroxyl value of from 200 to 1,000 g eq/10.sup.6 g and a hardener wherein the difference in gel time between the two polyesters is at least 3 minutes.
However, this resin composition for a powder coating is disadvantageous in that the specular gloss at 60.degree. of a coated face varies within a range of from several percent to 50% when a completely matte coated surface of a specular gloss at 60.degree. of 10% or less is to be formed. Therefore, a completely matte coated face having a stable specular gloss cannot be obtained from this resin composition.